The invention relates to gas and liquid fueled turbines and, more particularly, to combustors and a combustion liner cap assembly in industrial gas turbines used in power generation plants.
A combustor typically includes a generally cylindrical casing having a longitudinal axis, the combustor casing having fore and aft sections secured to each other, and the combustion casing as a whole secured to the turbine casing. Each combustor also includes an internal flow sleeve and a combustion liner substantially concentrically arranged within the flow sleeve. Both the flow sleeve and combustion liner extend between a double walled transition duct at their forward or downstream ends with a sleeve cap assembly (located within a rearward or upstream portion of the combustor) at their rearward ends. The flow sleeve is attached directly to the combustor casing, while the liner receives the liner cap assembly which, in turn, is fixed to the combustor casing. The outer wall of the transition duct and at least a portion of the flow sleeve are provided with air supply holes over a substantial portion of their respective surfaces, thereby permitting compressor air to enter the radial space between the combustion liner and the flow sleeve, and to be reverse flowed to the rearward or upstream portion of the combustor where the air flow direction is again reversed to flow into the rearward portion of the combustor and towards the combustion zone.
A plurality (e.g., five) of diffusion/premix fuel nozzles are arranged in a circular array about the longitudinal axis of the combustor casing. These nozzles are mounted in a combustor end cover assembly which closes off the rearward end of the combustor. Inside the combustor, the fuel nozzles extend into a combustion liner cap assembly and, specifically, into corresponding ones of the premix tubes. The forward or discharge end of each nozzle terminates within a corresponding premix tube, in relatively close proximity to the downstream end of the premix tube which opens to the burning zone in the combustion liner. An air swirler is located radially between each nozzle and its associated premix tube at the rearward or upstream end of the premix tube, to swirl the compressor air entering into the respective premix tube for mixing with premix fuel.
High combustion dynamics in a gas turbine combustor can cause disadvantages such as preventing operation of the combustion system at optimum (lowest) emissions levels. High dynamics can also damage hardware to a point that could result in a forced outage of the gas turbine. Hardware damage that does occur but does not cause a forced outage increases repair costs. Several corrective actions have been considered for reducing combustion dynamics in a gas turbine combustor. Tuning through fuel split changes, control changes and nozzle resizing have been tried with varying degrees of success. Often, a combination of these and other efforts is made to provide the best overall solution. Tuning and control setting changes are considered normal approaches to mitigating combustion dynamics as they are relatively simple changes to make when compared to other more costly and intrusive approaches such as changing hardware. Limitations do exist, however, as it is not only combustion dynamics that must be considered when tuning fuel splits or adjusting control settings. The effects on emissions (NOx, CO, and UHC), output, heat rate, exhaust temperature, fuel mode transfers, and turndown should all be considered when using these methods to mitigate dynamics and always involves a trade-off.
Nozzle resize is also an option sometimes used to deal with high dynamics but is typically reserved for use when the fuel composition has changed significantly from the design point. Also costly and time-consuming, this option has the disadvantage of having only a certain range of application based on the design pressure ratio range of the nozzle. A further change in fuel composition could once again require a different nozzle if the dynamics could not be tuned.
The design space is typically a last resort in dynamics mitigation at this stage due to the high cost normally associated with the development of a new piece of hardware. The goal is to lower dynamics without impacting the emissions, output, heat rate, exhaust temperature, mode transfer capability, and turndown that are often affected by the normal dynamics mitigation methods. For the most part, a more design oriented approach using small changes such as the cap modification decouples those parameters from the objective of reducing dynamics.